1. Field of the Invention
The present invention relates to a die for extruding a ceramic batch of raw materials of a ceramic honeycomb structural body, a method of producing the die, and a method of reclaiming the die.
2. Related Art Statement
A ceramic honeycomb structural body having a large number of parallel cells which are confined by relatively thin lattice like partition walls and extend therethrough in its axial direction, having an open frontal area of about 60-90% preferably 65-85%, and supporting a catalyst on the surface of the partition walls for removing harmful CO, NOx and HC from exhaust gases of automobile engines passing through the honeycomb structural body, is produced by extruding a ceramic batch of raw materials of the honeycomb structural body through a die to obtain a green honeycomb structural body, drying the green body, and firing the dried green body, as described in U.S. Pat. No. 3,824,196, for example.
Because the thus produced honeycomb structural body allows passages of exhaust gases that react with the supported catalyst, the honeycomb structural body must have a large reaction area and a low pressure loss for the exhaust gases passing therethrough. In order to increase the reaction area of a honeycomb structural body in a limited volume of a purifying apparatus for exhaust gas, preferably, the thickness of the partition walls is made as small as possible, while increasing the cell number per unit area of the honeycomb body. For increasing the cell number per unit area (the density of cells), if the dimension between the partition walls is solely decreased without decreasing the thickness of the partition walls, a large surface area for reaction with the exhaust gas can be obtain, but naturally the pressure loss of the exhaust gas becomes high to an untolerable extent, so that the use of such a honeycomb structural body is practically obstructed.
A honeycomb structural body having thin partition walls can be obtained by the use of an extrusion die having a small slit width. However, the slits of such an extrusion die have hitherto been produced by abrasion machining with the aid of a grinder, milling machining and an electric discharge machining, subjected to steel, so that the limit of the slit width, which is able to be formed by the above described machining with respect to slits having a given depth, is about 0.30 mm at the minimum.
The production of slits having a width smaller than 0.3 mm is very difficult due to the production accuracy of a grinder, a milling blade and a discharge electrode and due to the machining accuracy when the slits are formed by the use of these tools.
Japanese Examined Patent Application Publication No. 51-20,435 discloses that a honeycomb structural body having a round shape at the crossing corners of the partition walls causes the exhaust gas to flow smoothly and improves the reaction efficiency of the catalyst. This honeycomb structural body is produced by the use of an extrusion die having a round shape at the crossing corners of lattice-like slits, wherein the round shape is obtained by rounding off the crossing corners of the lattice-like slits by a cutting machining or electric discharge machining. However, the invention of the publication No. 51-20,435 has drawbacks in that even the formation of slits having a width of about 0.3 mm is difficult, and that after the slits having a width smaller than 0.3 mm are formed, it is extremely difficult to insert into the slits a cutting tool for rounding off the crossing corners of lattice-like slits of the die and to manufacture accurately a discharge electrode having small round portions.
As a result, a honeycomb structural body having partition walls of thickness of 0.3 mm or smaller than 0.3 mm or rounded crossing corners is difficult to obtain. Therefore, it is impossible to obtain a large catalytic reaction area and to cause the exhaust gas to flow smoothly. For example, when the honeycomb structural body is used as a catalyst carrier in the purification of exhaust gases of automobile engines, the honeycomb structural body is large in weight for use as a catalyst carrier and the catalyst requires a long time for a desired temperature increase at the starting period of the engine, and the exhaust gas can not be sufficiently purified during the starting period of the engine.
Even in precision machining the dimensional accuracy of slit widths always has a working error of about 10%. Thus, when it is intended to produce slits having a width of 0.3 mm for example, slits having a width of about 0.27-0.33 mm are obtained. When this tolerance is not accepted in practice, the extrusion dies have to be discarded. Further, when a slit having a width of 0.33 mm is obtained, and the upper limit of the slit width is 0.35 mm, only a very small number of honeycomb structural bodies can be extruded until the width 0.33 mm of the slits becomes 0.35 mm due to the wear of the slits.
Though it has been described in U.S. Pat. No. 3,930,522, column 6, lines 56-58, and U.S. Pat. No. 4,021,134, column 5, lines 46-50, that a die for extruding a honeycomb structural body is subjected to electroless nickel plating, these U.S. Patents only aim to prevent rusting of the die and to decrease the friction coefficient between the die and the ceramic batch, and neither discloses nor teaches the technical features and an effect of the present invention of controlling the slit width by controlling the thickness of the plating metal film, thereby to form slits having a width smaller than the width of slits formed by any conventional methods in a very simple way, and other distinguished effects attained in the present invention.
Further, even if a slit having a desired width has been obtained by electroless nickel plating, the plated nickel film is very poor in the abrasion resistance against the ceramic batch. The inventors have found out that, even when the slits having a nickel film plated thereon are heat treated at a temperature of 600.degree.-700.degree. C., which heat treatment temperature has hitherto been believed to be effective for improving the abrasion resistance of plated nickel film to the ceramic batch, the heat treatment is not effective at all for the improvement of abrasion resistance of the nickel film to the ceramic batch, and that hence the slits are worn by extrusions of a small amount of the ceramic batches, and as a result the slit width becomes larger than the specified width, and the die can not be used for the production of the honeycomb structural bodies and must be discarded. Of course, a nickel film having a thickness of about 0.01-0.05 mm, which is usually used for the purposes of preventing the rusting and lowering the friction coefficient, is worn and lost rapidly due to the abrasion by the ceramic batch so that it can not attain the above described purposes at all.
The life of dies can be prolonged by improvements of the abrasion-resistant property of the plated layer on the slits, and the abrasive-resistant property of the plated layer can be improved to some extent by varying the type of a reducing agent and subjecting the plated layer to a heat treatment. However, such improvements have been naturally limited, because the plating layer is composed mainly of nickel.
A method of improving the abrasive-resistant property of the electroless plating layer has been disclosed by Japanese Examined Patent Application Publication No. 47-24,857, wherein a composite plating layer composed of a plating layer and abrasive-resistant particles dispersed in the plating layer is applied on the surface of an article to be plated from an electroless plating solution containing dispersed abrasive-resistant particles.
However, when the above method is applied to dies for providing the composite plating layer, the following drawbacks are encountered, though the demand of high abrasive-resistant property can be satisfied.
Firstly, the reclaiming of such a composite plating layer was heretofore impossible in the technical field of the die, because of its poor solubility in a removing solution.
Secondly, if the plating layer on the slits of the die is constituted solely from a composite plating layer composed of a metal substrate and abrasive-resistant particles of such as silicon carbide, tungsten carbide or the like dispersed in the metal substrate, the plating layer has to be removed totally and the electroless composite plating has to be started from the beginning, when the plating layer of the die has to be reclaimed after the plating layer is worn by the abrasion of the ceramic batch. In this case, there are drawbacks in that the abrasive-resistant particles adhere firmly on the surface of the slits etc., so that a means has to be adopted of removing the adhered particles by high pressurized water, and that forcible removal of the particles leaves scars on the slits and/or the metal matrix of the die.
Thirdly, the electroless composite plating has a drawback of low deposition rate and requiring a deposition time of about 2-3 times of that of an electroless plating and a drawback of further necessitating a long deposition time when the slits surfaces are ground due to the repetition of reclaiming of the slits and an increased amount of electroless plating has to be effected on the ground surface of the slits.
Fourthly, if the electroless composite abrasive-resistant plating layer is directly deposited on the slits surface of the die, the amount of the electroless composite abrasive-resistant plating layer deposited on the corner portions of the matrix at the cross-section of the slits is smaller than that deposited on the other portion of the matrix, and therefore the surface of the deposited plating layer assumes a rough surface. Thus, a drawback occurs in that a uniform slit width is difficult to obtain. This is presumably due to a reason that the composite abrasive-resistant plating layer has a poor uniform deposition property as compared to the usual electroless plating layer, because the layer contains the abrasive-resistant particles uniformly distributed therein and the plating layer is easily influenced by the underlying matrix surface.
Fifthly, the outer configuration of the green honeycomb structural body shaped from the ceramic batch should be circular, elliptical or long elliptical shape in cross-section depending on the demands. In these cases, a mask plate having an opening corresponding to the shape of the green honeycomb structural body to be extruded has to be attached in front of the extruding die. If the mask plate closes a portion of the slits at the peripheral portion of the die depending on the shape of the opening of the mask plate, the closed portion of the slits at the peripheral portion of the die does not permit the ceramic batch to be extruded therethrough, so that the slits at the closed portion are not worn. Therefore, if the mask plate is exchanged to another mask plate having a different opening and used for extrusion of a ceramic batch, the resultant green honeycomb structural body has different thicknesses of the partition walls at the peripheral portion of the die from the thickness of the other portions of the die. Thus, it is extremely difficult to produce many green honeycomb structural bodies of various shapes in cross-section with an appropriate and uniform thickness of the partition walls by the use of a single die. Even if the wear of the slits of the die is small when the mask plates of various openings are used, reclaiming of the slits was heretofore impossible, so that the die has to be discarded in vain without reusing the valuable source material die.